Synthesis and Optimization of Visible Light Active BiVO4 Photocatalysts for the Degradation of RhB
Monoclinic BiVO4 powders were synthesized via a novel route using potassium metavanadate (KVO3) prepared by calcination of K2CO3 and V2O5 as a starting material and followed by hydrothermal treatment and were investigated for the degradation of Rhodamine B (RhB) under visible light irradiation. The synthesized BiVO4 particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and UV-Visible (UV-Vis) light diffuse reflectance spectrophotometry. The synthesis produced pure monoclinic BiVO4 particles with multimorphological features containing flower-like, flake-ball, flake, cuboid-like, and plate-like shapes and exhibited strong absorption in the visible light range. The BiVO4 prepared via KVO3 possessed excellent photocatalytic activity for the degradation of RhB under visible light. The performance of this catalyst was found to be superior to other BiVO4 photocatalysts prepared via ammonium metavanadate (NH4VO3) using coprecipitation, combustion, and calcination methods reported in literature, respectively.
BiVO4 is a promising semiconductor for use in solar photocatalysis due to its efficient visible light absorption and other favorable properties such as its nontoxicity [1, 2]. BiVO4 exists in three major crystal phases, namely, monoclinic scheelite, tetragonal zircon, and tetragonal scheelite [3, 4]. Of these, monoclinic BiVO4 possesses a band gap of 2.4 eV, making it the most appropriate BiVO4 phase for applications to visible-light-induced photocatalysis [5, 6]. Solid state reactions are traditionally used for the synthesis of BiVO4 [7, 8]; however other synthesis methods such as sonochemical routes [9, 10], room temperature aqueous processes , molten salt methods , hydrothermal processes [12, 13], chemical bath depositions [14, 15], organic decompositions [16, 17], combustion synthesis , and coprecipitation processes  have been recently proposed and investigated.
In order to enhance the photodegradation efficiency of the composite, various BiVO4 morphologies such as flower-like , flake-ball, flake , cuboid-like, and plate-like  structures exhibiting enhanced surface area for photocatalysis have been reported. However, most of the synthesis methods investigated cannot result in the synthesis of BiVO4 materials with both high crystallinity and high surface areas required for efficient photocatalytic reactions [9, 18]. In order to address these issues and to optimize the photocatalytic properties of monoclinic BiVO4, certain research efforts have been focused on employing templates and surfactants such as silica (KIT-6)  and CTAB  and CMC  and starting materials made from organic polymers and inorganic salts . However, the generated impurities from these methods may be responsible for the blockage of active sites on BiVO4 particles, decreasing the overall photocatalytic activity.
In recent years, potassium vanadium oxides, particularly KVO3 and K3VO8, have been reported as promising alternative starting materials for the preparation of monoclinic BiVO4 [24, 25]. For example, Kelmers  prepared KVO3 precursor by dissolving equimolar amounts of K and V, as KOH and V2O5, respectively. KVO3 was also previously prepared using KCl and V2O5 in the presence of steam . Liu et al.  reported using NaVO3 and V2O5 as two different vanadium sources to synthesize BiVO4 particles via a hydrothermal method (at synthesis conditions of 140°C from NaVO3 and 200°C from V2O5), to obtain two different morphologies of BiVO4 particles with a plate form and rod-like shapes, respectively.
In this work, a novel method for the preparation of monoclinic BiVO4 via KVO3 starting material prepared by facile calcination of K2CO3 and V2O5 was investigated. BiVO4 synthesized via this route and subsequent hydrothermal synthesis was compared to a similar hydrothermally prepared BiVO4 obtained via NH4VO3 starting material. In addition, BiVO4 synthesized was compared to BiVO4 synthesized by other methods reported in literature, such as coprecipitation, combustion, and calcination. Compared to BiVO4 obtained by all other synthesis methods, the as-prepared BiVO4 synthesized via the novel route proposed exhibited enhanced photocatalytic activity for the degradation of model organic pollutant, Rhodamine B (RhB), under visible light irradiation due to its desirable phase structure, morphologies, and surface areas. There have been no reports to date on the synthesis of monoclinic BiVO4 powders via KVO3 and the hydrothermal method; therefore the study of this material and its enhanced performance when prepared via this route contributes to the advancement of visible light photocatalysis using novel bismuth-based structures.
2.1. Catalyst Synthesis
All chemicals used were of analytical purity. BiVO4 was synthesized in a two-step procedure, where potassium metavanadate (KVO3) was first prepared by calcination, followed by the hydrothermal synthesis of monoclinic BiVO4 particles. In the first process, 0.38 g K2CO3 (Fisher Scientific, Certified ACS) and 0.5 g V2O5 (ACROS Organics, 99.6%) were dissolved in 35 mL deionized water (DW) under vigorous magnetic stirring. The resulting red solution was then poured into an evaporation dish and dried at 50°C in an oven overnight. The dry sample was then ground and annealed in air at 457°C (730 K) for 5 h, resulting in a pink KVO3 powder. To prepare BiVO4, 1 mmol Bi(NO3)3·5H2O and 0.1056 g of the prepared KVO3 powder were mixed in 35 mL DW and allowed to react under continuous stirring for 50 min to obtain a vivid yellow slurry. The pH of the slurry was then adjusted by adding 50 vol.% acetic acid, until a final pH of ~1 was reached. The slurry was then transferred into a 45 mL Teflon-lined stainless steel autoclave and the reaction was allowed to proceed at 200°C for 24 h. The resulting solid was collected by filtration, washed with DW several times, and then dried in air overnight.
To investigate the influence of starting materials on the photocatalytic activity of BiVO4 prepared via the hydrothermal method, two hydrothermal synthesis routes were investigated, namely, via KVO3 and via NH4VO3, respectively, as outlined in Table 1. In order to compare the morphologies, optical properties, and photocatalytic activities of BiVO4 obtained by the proposed hydrothermal synthesis to properties of BiVO4 prepared via various other syntheses reported in the literature, a number of alternative synthesis methods were also explored, as outlined in Table 2. The nomenclature for the as-prepared BiVO4 samples is given in Table 1 for two samples prepared via hydrothermal synthesis and Table 2 for samples prepared via other synthesis methods. It should be noted that the operating parameters selected for the various syntheses methods shown in Table 1 were adopted from the optimal parameters reported in the corresponding literatures.
X-ray powder diffraction (XRD) measurements were carried out on Rigaku Ultima IV, in Bragg-Brentano geometry, using Cu Kα1 ( nm) radiation operating under 40 kV and 40 mA, and with a scanning range of 2θ from 10° to 70°. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-7500F field emission SEM operated at 2.00 kV. X-ray photoelectron spectroscopy (XPS) was conducted on Kratos Analytical Axis Ultra DLD instrument with monochromated Al X-rays at 140 W. The powder UV-Vis diffuse reflectance spectra (DRS) were recorded on Thermo Evolution 300 UV/Vis spectrophotometer equipped with Praying Mantis diffuse reflectance accessory, and the spectra were collected at a scan rate of 240 nm min−1.
2.3. Photocatalytic Activity
Photocatalytic performance was quantified by the decomposition of RhB (Sigma-Aldrich) as a model organic pollutant under visible light irradiation. A slurry reactor was placed in a reflective housing to prevent outside light from entering the reactor and inside light from exiting the system. 300 W ELH tungsten halide bulb (Ushio) was used as a light source with a 410 nm cut-off filter (Kenko Zeta, nm, transmittance > 90%) to provide visible light irradiation. The light source was placed at a distance of 15 cm from the top of the slurry. The corresponding irradiation was measured using a quantum meter (Biospherical QSL-2100; 400 nm < < 700 nm) and was found to be approximately 4.7 × 10−3 Einstein m−2 s−1. Cooling was provided by an external cooling jacket, and temperature of the reaction was controlled to 22°C ± 2. Before illumination, 0.1 grams of photocatalyst dispersed into 150 mL of RhB solution (5 mg L−1) was allowed to reach adsorption/desorption equilibration under continuous magnetic stirring at 340 rpm for 30 min in the dark. Irradiation was then provided for 2 h for each photocatalytic degradation trial. Samples were withdrawn at 20 min time intervals and separated by centrifugation at 10 × 103 rpm for 3 min in accuSpin Micro 17 (Fisher Scientific) microcentrifuge to remove the suspended catalyst, and the supernatant fluid was analyzed by monitoring the peak absorbance ( nm for RhB) via a Genesys 10-UV spectrophotometer (Geneq Inc.). A standard curve for RhB was prepared and the concentration was determined by the measured absorbance and the Beer-Lambert Law. UV/Vis analysis of RhB samples was obtained via Biochrom Ultrospec 60 UV/Vis spectrophotometer. Quenching tests were performed by the addition of appropriate reactive species scavengers. 2 mg Benzoquinone (BQ) (reagent-grade ≥ 98%, Sigma-Aldrich) was used to trap superoxide radicals (), 0.15 g ammonium oxalate (AO) (ACS, Sigma-Aldrich) was used as the holes scavenger, and 3 mL tert-butyl alcohol (TBA) (ACS, Sigma-Aldrich) was used to trap hydroxyl radicals (), respectively.
3. Results and Discussion
3.1. XRD Analysis
The XRD pattern for the KVO3 material obtained by calcination of K2CO3 and V2O5 is shown in Figure 1. The diffraction peaks of KVO3 could be well indexed to the orthorhombic phase (JCPDS Card number 00-033-1052).
As shown in Figure 1, the XRD pattern for the prepared KVO3 was well indexed to the reference pattern, indicating that pure KVO3 particles with orthorhombic structure resulted from calcination of K2CO3 and V2O5 in air at 457°C (730 K) for 5 h.
XRD patterns of BiVO4 obtained via various synthesis methods are shown in Figure 2. For all the prepared photocatalysts, all of the diffraction peaks could be well indexed to the monoclinic scheelite phase of BiVO4 (JCPDS Card number 01-083-1699) with the exception of BiVO4 (H-140-8) synthesized via hydrothermal method, which was indexed to a tetragonal structure (JCPDS Card number 00-014-0133) but also exhibited monoclinic reflections.
The major reflections observed for the monoclinic BiVO4 structure occurred at 28.8° and 28.9°, and the major diffraction peak of the tetragonal BiVO4 structure (JCPDS Card number 00-014-0133) occurred at 24.4°. As shown in Figure 2, the XRD pattern of BiVO4 (H-140-8) possessed mixed phases of both tetragonal and monoclinic structures, as evidenced by the characteristic peaks observed at both 24.4° and 28.8°, respectively. According to the relative intensities of these reflections, the tetragonal structure was thought to be the dominant phase present. All other prepared samples exhibited pure monoclinic structure and were well indexed to the expected reflections according to JCPDS Card number 01-083-1699. BiVO4 sample prepared via coprecipitation (Co-pre-350-24) was observed to possess lower crystallinity due to the long calcination process used. Sintering was thought to occur at 350°C when the sample was annealed for 24 h, which caused the growth of crystalline sizes and consequent decrease of surface area . It was also previously reported in literature that low crystallinity could lead to an increase in the amounts of defects formed relative to the crystalline quantities produced . This low crystallinity and large particle size was found to lead to lower photocatalytic activity [31, 33]. From the results obtained, the simple calcination process could not be used for the preparation of highly crystalline BiVO4 materials. In contrast, BiVO4 prepared by combustion and calcination (Comb-500-3 and Calc-450-5, resp.) possessed higher crystallinities. The XRD patterns obtained from BiVO4 samples prepared via hydrothermal synthesis (H-140-8 and C+H-200-24, resp.) indicated that the resulting crystalline structures were dependent on the temperature and time used in the synthesis. However, BiVO4 (C+H-200-24) sample prepared via the novel route proposed using KVO3 as the starting material resulted in the formation of a highly crystalline, monoclinic structure which was thought to be advantageous for photocatalysis. The pattern obtained also suggested that higher temperatures and longer reaction times were favorable for the formation of monoclinic BiVO4.
The crystalline size was estimated from the Scherrer formula  as follows:where is the crystalline size; is the wavelength of the X-ray radiation ( nm); is the sphericity constant (usually taken as 0.9); is the peak width at half-maximum height of the sample after subtraction for equipment broadening. In , the values of were 28.95° for monoclinic BiVO4 and 24.37° for tetragonal BiVO4, respectively. The crystalline sizes of BiVO4 prepared via various syntheses are shown in Table 4.
The molar ratio of bismuth to vanadium was thought to influence the formation of monoclinic BiVO4. Therefore, the molar ratio of Bi : V was kept at 3 : 2 during hydrothermal synthesis (C+H-200-24) to ensure the formation of monoclinic BiVO4 and provide sufficient vanadium for the stoichiometric conversion from tetragonal to monoclinic BiVO4 . On the basis of our experiments, relevant chemical reactions were formulated as follows:
According to Kudo et al.  and Liu et al. , the formation of BiVO4 was sensitive to temperature and pH of the aqueous medium. The authors indicated that high temperature (~200°C) might facilitate the conversion of excess BiO+ to the monoclinic structure, by the adsorption on the tetragonal BiVO4 crystals. In addition, under pH ~1, the interaction of BiO+ cations with present in KVO3 was thought to be much stronger than with in NH4VO3, due to Coulombic attractions.
The increase of the KVO3 solubility in water is greater than that of NH4VO3 over a temperature range of 293 K to 323 K , which may have also contributed to providing more for the generation of BiVO4. Furthermore, in KVO3 was reported to be converted to an intermediate product of vanadate ions (i.e., ) during the hydrothermal process by increasing temperature, to eventually form a stable KV3O8 phase , as shown in and . This phase could serve as an inorganic morphology control agent during the synthesis .
The pH value was also thought to influence growth process and the morphology of BiVO4 particles, and the pH for the precursor slurry of BiVO4 was ~1 due to the hydrolyses of Bi(NO3)3·5H2O . The final pH of BiVO4 precursor slurry was kept at ~1 by adding 50 vol.% acetic acid (acidity = 4.76). Acetic acid was thought to play the following roles during the synthesis:(1)Adjusted concentrations between Bi(III) and V(V), acting as a morphology controlling agent for BiVO4 .(2)Promoted the conversion from tetragonal BiVO4 to monoclinic BiVO4 crystals resulting in pure monoclinic BiVO4 particles.(3)Preferentially formed pure monoclinic BiVO4 particles with nanoscaled average crystalline sizes .
3.2. SEM Analysis
The microstructure and morphology of synthesized BiVO4 particles were investigated by SEM, and the images obtained are shown in Figures 3 and 4.
From Figures 3(a), 3(b), and 3(c), the as-obtained samples prepared using various methods were observed to be quasi-spherical with 100 nm nanoscale length. As shown in the inset of Figure 3(a), clusters of sintered particles were distinctly observed for the sample prepared via coprecipitation instead of individual balls, and this was thought to be due to severe sintering, as expected from the XRD results obtained (Figure 1 BiVO4 (Co-pre-350-24)). Comparison of Figures 3(a), 3(b), and 3(c) indicated that the synthesized BiVO4 particles exhibited good morphology evolution which was mainly attributed to shorter calcination times at higher calcination temperatures [24, 38]. From Figure 3(b), the morphology was observed to be composed of nanoscale balls without severe sintering. However, because of the drawbacks associated with calcination synthesis, the surface area of BiVO4 particles formed using the combustion method declined to some extent, which was thought to negatively influence the photocatalytic activity. As seen in Figures 3(d) and 3(e), excellent flake-ball superstructures could be observed for the samples prepared by hydrothermal synthesis. As shown in the inset of Figure 3(d), the morphological evolution of BiVO4 particles from nanoscale flakes and squares to balls with rough surfaces was likely due to the effect of the hydrothermal aqueous system, in which the strong surface-charge driven forces influenced the resulting photocatalyst structures [39, 40]. As the treatment time increased, part of these nanoscale crystals agglomerated to balls. As such, shorter reaction times were thought to provide inadequate surface-charge driven forces for the formation of smooth balls in the hydrothermal processes. Nevertheless, the surface area of BiVO4 particles rapidly increased with treatment time, and multimorphological features of BiVO4 particles were distinctly observed. If various reaction times were used in conjunction with different temperatures, multimorphological features of BiVO4 particles were reported to be more diverse in one system . For instance, flake-ball superstructures with obvious edges were observed in Figure 3(e) (see Figure 3 inset), which agreed well with BiVO4 monoclinic structure seen in the XRD pattern (Figure 2, BiVO4 (C+H-200-24)).
To investigate BiVO4 (C+H-200-24), additional SEM imaging was performed, and the results are shown in Figure 4. The multimorphological features observed in the superstructure are highlighted in the insets, where insets 1, 2, and 3 indicate flower-like , flake-ball and flake , cuboid-like, and plate-like structures , respectively, which were present in the same system. Iwase and Kudo  reported that the edge surfaces of the plate-like and well-crystallized particles of monoclinic BiVO4 were highly active sites for photocatalytic oxidation reactions. Therefore, monoclinic BiVO4 particles with multimorphological features were thought to be advantageous for photocatalysis due to the increased surface area, resulting in promoting adsorption of dyes onto the photocatalysts and their subsequent photodegradation.
3.3. XPS Analysis
In order to explore the chemical and electronic states of BiVO4, XPS analysis of a representative sample (BiVO4 (C+H-200-24)) was performed, and the results from the full-scan spectrum are shown in Figure 5(a). The corresponding high resolution XPS spectra patterns are presented in Figures 5(b), 5(c), and 5(d), for Bi 4f, V 2p, O 1s, and C 1s states, respectively. The spectrum of the C 1s was attributed to the adsorbed carbon present in the samples.
From Figure 5(b), the spectra of Bi species were seen to exhibit two symmetric peaks centered at 158.7 eV and 164 eV (for spin-orbit splitting characteristic of Bi 4f7/2 and Bi 4f5/2, resp.) which corresponded to characteristic signals of Bi3+ [42–44]. The observed V 2p3/2 peak (Figure 5(c)) was composed of two components at binding energies of 516.8 eV and 517.2 eV, which were assigned to V4+ and V5+ species, respectively, in terms of V 2p3/2 orbital [43, 45]. Interestingly, as shown in Figure 5(c), the difference in intensity for those two species was small. Accounting for electroneutrality, BiVO4 (C+H-200-24) had an apparent oxygen deficiency filled with V4+ species  incorporated with V5+ species on the surface of BiVO4 in dynamic equilibrium, in which the lost oxygen groups were thought to be consumed to create extra hydroxyl groups on the surface of BiVO4, and these hydroxyl groups generated may have caused the presence of oxygen vacancies . The amount of lost oxygen was determined by the molar ratio of V4+/V5+ in the whole system [43, 45, 47]. Furthermore, the peak at a binding energy of 524.1 eV was attributed to V 2p1/2 orbital. As seen in Figure 5(d), the O 1s spectral peak located at 530 eV was composed of two separate peaks at binding energies of 529.5 eV and 532.1 eV. The former peak was characteristic of oxidic species such as O−, , or O2− in an effective oxide overlayer  and was attributable to oxygen vacancies in BiVO4−δ . The latter O 1s peak was assigned to lattice oxygen of BiVO4 crystallites, resulting from interfacial hydroxyl groups (i.e., OH radicals) or molecularly adsorbed water in the system [44, 48, 49].
BiVO4 is a direct band gap semiconductor [6, 42], which is favourable for retaining low energy direct transitions. In addition to the crystal structure observed, the electronic structure of a semiconductor material affects the photocatalytic activity [50, 51]. For monoclinic BiVO4, as n-type material , V 3d orbitals preferentially formed the conduction band (CB), whereas Bi 6s and O 2p orbitals favoured the composition of a hybridized valence band (VB) , where charge transfer occurred from Bi 6s and O 2p hybrid orbitals to V 3d orbitals under photoexcitation. The hybridization of Bi 6s and O 2p levels created a desirably dispersed VB, which could enable the mobility of photoexcited holes , resulting in a high activity of photocatalytic oxidation towards organic pollutants .
3.4. Optical Properties of BiVO4
As seen in Figure 6, the UV-Vis spectra of all BiVO4 samples exhibited intrinsic absorption in the visible light region attributed to the characteristic features of BiVO4 (both monoclinic and tetragonal phases), which was related to the band-band transition of a semiconductor with a direct band gap [6, 31, 52].
The band gaps of the prepared samples were estimated from the DRS data, where respective band gap energies (, eV) were estimated using the following equation: where is the maximum wavelength of absorption by photocatalysts (nm) and is the estimated band gap of photocatalysts (eV).
The calculated band gap energies were 2.38, 2.30, 2.39, 2.43, and 2.38 eV for BiVO4 (Co-pre-350-24), BiVO4 (Comb-500-3), BiVO4 (Calc-450-5), BiVO4 (H-140-8), and BiVO4 (C+H-200-24) samples, respectively. All of BiVO4 particles prepared via various synthesis methods had strong absorption in the visible light range ( nm). From the SEM presented in Figure 3(b), BiVO4 (Comb-500-3) sample had the largest agglomerate size due to the severe sintering, and this was thought to have caused the band gap of 2.30 eV observed. In contrast, BiVO4 (Co-pre-350-24), BiVO4 (Calc-450-5), BiVO4 (H-140-8), and BiVO4 (C+H-200-24) samples were all present as smaller particles and exhibited band gaps of 2.38, 2.39, 2.43, and 2.38 eV, respectively. The band gap of a semiconductor has been reported in literature to increase with an absorption shift to shorter wavelengths due to decreased individual particle size [53, 54]. In accordance with this, a decrease in the light absorption was observed for BiVO4 (H-140-8) and BiVO4 (C+H-200-24) and was thought to be due to the formation of flakes of several hundred nanometers in diameter on the surface of BiVO4 (Figures 3 and 4), which caused the scattering of light .
3.5. Photocatalytic Activity of BiVO4
3.5.1. Photodegradation of RhB
RhB is a basic red xanthene dye with a high water solubility and a molecular formula of C28H31ClN2O3 . The structure of RhB is shown in Figure 7 for reference.
RhB is widely applied as a model target organic pollutant to study photoactivity. The results of visible-light-induced photoactivity in the presence of various prepared BiVO4 samples are shown in Figure 8.
To investigate the self-degradation of RhB, a control test for photolysis in the absence of BiVO4 samples was performed under visible light irradiation. This self-degradation was found to contribute up to 9.6% in 2 h of irradiation (Figure 8) and as such may have induced a self-sensitized process. However, compared to the results obtained in the presence of BiVO4 samples, the degree of RhB self-degradation due to photolysis was thought to have a relatively low contribution to the overall degradation observed in the presence of a photocatalytic process.
In contrast, the RhB degradation by BiVO4 (C+H-200-24) was up to 88% in 2 h and represented a higher removal capacity than both photolysis and the other BiVO4 samples prepared, where degradation of 7.2%, 6.9%, 13%, and 23% corresponding to BiVO4 (Copre-350-24), BiVO4 (Comb-500-3), BiVO4 (Calc-450-5), and BiVO4 (H-140-8), respectively, was observed, as shown in Figure 8. This indicated that BiVO4 (C+H-200-24) was comparatively more active than other BiVO4 samples, and the increase in activity observed was thought to be due to the optimal synthesis method (calcination and hydrothermal) and optimal starting material (KVO3) used in the hydrothermal process. These factors acted in concert to create an overall optimized catalyst, which exhibited improved adsorption/desorption kinetics of RhB on the surface of BiVO4 particles, as well as an improvement in the visible light absorption and photocatalytic degradation.
In addition to the photoactivity observed, the adsorption of RhB by the prepared BiVO4 samples also influenced the degradation. As a cationic dye in acidic media, the adsorption of RhB onto a photocatalyst required the presence of anionic catalysts with negative surface charges in order to facilitate photodegradation . As n-type material , BiVO4 was a desirable anionic catalyst with respect to the photodegradation of RhB, because it could adsorb the RhB molecules by Coulombic attractions and also provided extra electrons and created an excess of negative (n-type) electron charge carriers.
3.5.2. Langmuir-Hinshelwood Kinetics
To quantitatively compare the powders synthesized using various conditions, Langmuir-Hinshelwood kinetic analysis was applied to RhB degradation in the presence of BiVO4 particles. Here the simplification to a pseudo-first-order reaction model is given by the following:where and are the initial RhB concentration (mol L−1) and concentration at reaction time (min), respectively. is the reaction rate constant (mg (L·min)−1), is the adsorption coefficient of the reactant onto catalyst (L mg−1), is pseudo-first-order rate constant in Langmuir-Hinshelwood expression (min−1), and is the irradiation time (min). The reaction is expressed by the pseudo-first-order equation if adequately dilute initial concentrations are used ( < 10−3 mol L−1) . In our experiments, was adequately small ( = 1.04 × 10−5 mol L−1). Hence, could be used to describe the pseudo-first-order rate constant and was obtained from the gradient of the line of versus time . The pseudo-first-order constant was used as a criterion for the comparison of the photoactivity of the catalysts.
Taking the initial 20 min of degradation into account, the comparison of rate constants obtained is given in Table 3.
As seen from Table 3, all samples exhibited photocatalytic activity for the degradation of RhB to some extent. Of these prepared samples, BiVO4 (C+H-200-24) showed the highest degradation rate of . Therefore, the pseudo-first-order rate constant of BiVO4 (C+H-200-24) agreed well with the corresponding result of photocatalytic degradation shown in Figure 8.
3.5.3. Apparent Photonic Efficiency
It has been reported that apparent photonic efficiency can be applied to determining the effectiveness of photon utilization. The initial reaction within 20 min was considered to be linear, and the photonic efficiency at 20 min was taken as the reference for comparsion of photocatalytic activities among BiVO4 samples. The formula for apparent photonic efficiency is given according to the following :where is the apparent photonic efficiency (i.e., the ratio of reaction rate and incident light intensity), is the solution volume (m3), is the change in the concentration (mol m−3), is the flux of photons (Einstein m−2 s−1), is the illuminated area (m2), and is the change in time (min).
The results of photonic efficiency agreed well with the first-order kinetic constants obtained through Langmuir-Hinshelwood analysis. BiVO4 (C+H-200-24) was found to possess superior photonic effiency to the other samples, with the maximum photonic efficiency of 0.0131. The calculated values of apparent photonic efficiencies, crystalline size, and band gap of prepared catalysts are shown in Table 4.
The results indicated that BiVO4 (C+H-200-24) via optimized hydrothermal method gave the maximum value of 0.0142 min−1 corresponding to the highest degree of photocatalytic degradation of RhB, as shown in Figure 8. Table 3 also indicated that the rate constant of BiVO4 (C+H-200-24) sample was higher than the other samples prepared via hydrothermal method, indicating that the starting materials affected the overall photocatalytic activity observed.
3.5.4. RhB Degradation
In order to investigate the RhB photocatalytic degradation, the changes of UV-Visible spectrum during the decolorization of RhB under visible irradiation were investigated, and the results are shown in Figure 9.
As seen in Figure 9, the intensity of characteristic absorption peak at 552 nm diminished rapidly with time and had gradually hypsochromic shifts of absorption bands from 552 nm to 535 nm, in agreement with the results reported in literature [60, 61]. The decline of RhB at its maximum absorption wavelength was attributable to the cleavage of the whole conjugated chromophore structure, while the small shifts from 552 nm to 535 nm implied the stepwise N-deethylation of RhB. Generally, those two competitive pathways were thought to occur throughout the whole degradation process. However, from Figure 9, the high initial concentration of RhB under visible light irradiation may have slowed the production of N-deethylated intermediates. The main reason may be due to the carboxyl group preferentially adsorbing on the surface of BiVO4, rather than that of the diethyl-amino group at the weak acidic conditions , from which the active oxygen species mainly attacked the conjugated chromophore ring structure and resulted in the cleavage of the ring structure of RhB in the presence of BiVO4. Therefore, the observed absorption shifts were thought to be relatively negligible compared to the decrease in peak absorbance due to the cleavages of the whole conjugated chromophore. Further decomposition of the deethylated intermediates was thought to occur with longer and continuous treatment under visible light irradiation, as reported in literature .
3.5.5. Recyclability and Durability of Photocatalytic Activity for BiVO4 Particles
Recyclability of photocatalyst is an important factor for the assessment of practical utilization and scalability of the catalyst. For the recyclability runs, the photocatalyst were separated by centrifugation without washing after each run, and the degraded RhB supernatant was removed, and fresh RhB solution was added. To assess the recyclability and durability of BiVO4 (C+H-200-24) sample for photocatalytic degradation, four runs of recycling experiments were conducted, and the results are shown in Figure 10.
From the results obtained, the concentration of RhB was seen to obviously decrease in each cycle, implying that BiVO4 (C+H-200-24) could exhibit some degree of visible-light-induced photoactivity in four successive cycling experiments without regeneration. Despite this, some loss of photoactivity after recycling was observed, and the activity in the first run was greater than that observed in the sequential uses at the end of 2 h visible light irradiation for the second to fourth run, respectively. The 45% degradation of RhB observed in the final run indicated that some activity was still maintained by the catalyst, although active sites were thought to be possibly blocked by the intermediates from decomposed RhB [63, 64].
3.5.6. Role of Reactive Species
The photocatalytic degradation of organic pollutants is generally induced by reaction with reactive species such as h+, , and , which are generated on the surface of photocatalysts upon irradiation . To further investigate the impact of various reactive species on the photocatalytic degradation reactions, the influence of numerous additives was studied on photodegradation of RhB. Reactive species scavengers (including BQ, AO, and TBA) were employed, and their effect on suppressing photocatalytic degradation of RhB in the presence of BiVO4 was studied. The results are shown in Figure 11.
From Figure 11, it was found that, without scavengers, the photocatalytic degradation of RhB on BiVO4 (C+H-200-24) was ~88% after 2 hours. Two mg BQ as a scavenger of was added in the photocatalytic system [65, 66] causing the photodegradation of RhB on BiVO4 (C+H-200-24) to decrease significantly, reducing the degradation to 17%. This suggested that played a significant role in the photocatalytic degradation mechanism of RhB over BiVO4. 0.15 g of AO was used as a hole scavenger [67, 68]. This addition caused a degradation of RhB up to 75%, with little change from the unquenched test. Three mL TBA was added into the photodegradation system as scavenger  and resulted in 71% RhB degradation. From the results of the quenching tests, the degradation of RhB by BiVO4 under visible light irradiation was thought to be mainly attributed , where and holes species played a lesser role in the process.
3.5.7. Mechanism of Photocatalytic Activity
To characterize the photodegradation pathways of the dye pollutant in the absence and presence of BiVO4 under visible light irradiation, considering the results obtained from the photolytic and photocatalytic degradation study, as well as the quenching tests, respectively, the following scheme was proposed: or radicals peroxides or hydroxylated ⋯ intermediates degraded or mineralized products [57, 70].
In accordance with reactions – and Figure 12, the amount of adsorbed RhB and surface-adsorbed molecular oxygen were significant parameters which were dependent on the degree of initial excitation of dye molecules and the yield of molecular oxygen such as and , which were assumed to be responsible for the dye degradation. From the viewpoint of enhancing photocatalytic degradation of RhB, an optimized photocatalyst with higher adsorption/desorption kinetics of dye was also necessary.
In our research, the optimized BiVO4 (C+H-200-24) was able to satisfy these requirements due to its high crystallinity, lower defects content, lower band gap of 2.38 eV, and multimorphological features resulting in high visible-light-induced photocatalytic activity for RhB degradation. In addition, reactions and were presumed to occur simultaneously, because BiVO4 particles with a direct band gap around 2.4 eV  could be easily excited by visible light. Concurrently, the electron injected from the excited into the conduction band of BiVO4 facilitated the stepwise degradation of , while the injected electron on the conduction band reduced surface-adsorbed oxidants (e.g., O2) . Meanwhile, reactions and occurred sequentially due to sufficient high-energy e− and h+ presented in the system. Furthermore, self-photosensitization of dye occurred during the whole visible-light-induced photodegradation process via oxidizing radicals , implying that photosensitization was a possible route due to the photolysis. Despite this, the optimized BiVO4 (C+H-200-24) particles exhibited a much higher overall capability for photocatalytic activity presented in Figure 8, where BiVO4 particles acting both as electron donors and as electron carriers played a critical role in transporting electrons to acceptors in order to achieve photocatalytic reactions under visible light irradiation, and these holes and electrons reacted with adsorbed RhB both directly and indirectly, improving the photocatalytic degradation of RhB.
Fine monoclinic BiVO4 particles were synthesized through a facile hydrothermal method using the synthesized potassium metavanadate (KVO3) as precursor. The synthesized BiVO4 (C+H-200-24) particles had a high crystallinity and multimorphological features, and they exhibited good visible-light-induced photocatalytic activity for the degradation of RhB. A maximum pseudo-first-order degradation rate of 0.0142 min−1 was observed for BiVO4 (C+H-200-24). Comparison of the photoactivity observed using samples prepared by various synthesis methods and conditions indicated that adsorption/desorption kinetics, particle size, crystallinity, morphology, and optical properties of photocatalysts all influenced the photoactivity obtained. The hydrothermal synthesis promoted the reaction between Bi(NO3)3·5H2O and KVO3, and the interaction of BiO+ cations with from KVO3 was thought to be much stronger than that from NH4VO3 due to the electrostatic forces present during the synthesis. Additionally, the hydrothermal method facilitated the production of more interfacial radicals on BiVO4 particles, promoting reactions between electrons (or holes) and the adsorbed species on BiVO4 particles such as adsorbed O2 and H2O, in order to prevent photoexcited electrons and holes from recombining, facilitating photocatalytic decomposition.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would like to thank the Center for Catalysis Research and Innovation and the Department of Earth Sciences at the University of Ottawa, for help with sample characterization, and the Chinese Scholarship Council for the financial support.
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